Conventionally, there have been various lighting units proposed including a light guide and an LED light source in the technical field of vehicular lighting units (for example, see Japanese Patent No. 4339028 or U.S. Pat. No. 7,070,312 correspond to the JP patent).
FIG. 1 shows a lighting unit 90 described in Japanese Patent No. 4339028, which can include a transparent resin light guide 91 and an LED light source 92.
The light guide 91 can be configured such that light emitted from the LED light source 92 can enter the inside of the light guide 91, be reflected off the front surface 91a and reflected off the rear surface 91b, thereby being projected forward from the front surface 91a. 
The lighting unit 90 has the front surface 91a of the light guide 91 being a plane surface and the rear surface 91b opposite thereto being a continuous surface (for example, revolved paraboloid), and accordingly, the thickness between the front and rear surfaces 91a and 91b becomes large. This may increase the molding time for the light guide 91 and the amount of a transparent resin material, thereby resulting in cost increase. In general, the molding time for a molded article may be proportional to the square of the thickness of the molded article.
In addition, when the thickness is large, shrinkage or the like giving adverse effects on the accuracy of the light guide 91 (by extension, light distribution) may be likely to occur. There may be another problem due the large thickness (namely, the optical path length in the light guide 91 may be longer) wherein the light entering the light guide may be likely to be affected by the absorption of the transparent resin material or haze (volume scattering). In order to reduce such adverse effects like the absorption of the transparent resin material or haze (volume scattering), it has been a consideration to shorten the optical path length in the light guide 91. However, this has been achieved by miniaturization of the entire size of the light guide 91, resulting in decrease of the light utilization efficiency and the like.
Further, the lighting unit 90 as described above may have a problem of lower degree of freedom with regard to the formation of light distribution because the rear surface 91b of the light guide 91 is a continuous surface (revolved paraboloid, for example). In order to cope with this problem, a plurality of lighting units 90 each forming different light distribution are combined to synthesize a desired light distribution pattern as disclosed in the above patent literature.